Devices and methods for detecting at least one fraction of a gas component of a gas in a measuring gas chamber are believed to be understood from the related art. For example, the gas may be an exhaust gas of an internal combustion engine, in particular in the automotive field, and the measuring gas chamber may be an exhaust system, for example. The device may be a lambda sensor in this case, for example.
Such lambda sensors are discussed, for example, in Robert Bosch GmbH: Sensoren im Kraftfahrzeug (Sensors in Motor Vehicles), 2007 edition, pages 154-159. Lambda sensors, in particular universal lambda sensors, set two substance flows, in particular oxygen flows, in equilibrium between a cavity of the device and the measuring gas chamber. One of the substance flows is driven in this case by concentration differences via a diffusion barrier. A further substance flow is driven via a solid electrolyte and two electrodes, in particular two pump electrodes, controlled by an applied pump current. The pump current may be regulated in such a way that a constant and very low oxygen concentration results in the cavity. A concentration profile via the diffusion barrier is unambiguously determined by a constant regulating point in the cavity, in particular a constant setpoint voltage resulting in an oxygen concentration, and by an exhaust-side oxygen concentration. An inflow of oxygen molecules from the measuring gas chamber to the cavity results in accordance with this unambiguous concentration profile and corresponds to the regulated pump current. This pump current may therefore be a measured value for the oxygen concentration in the measuring gas chamber, in particular for the oxygen concentration applied on the exhaust side.
In particular two different variants of lambda sensors are believed to be understood from the related art: lambda sensors having two cells and lambda sensors having only one cell.
Lambda sensors having two cells are discussed, for example, in DE 4410016 C2. An oxygen detection device is discussed therein for detecting an oxygen concentration of a measuring gas, including a first electrochemical cell having a reference electrode and having a measuring electrode, and a second electrochemical cell having an electrode pair.
In such sensors having two cells, the first electrochemical cell is usually integrated as a measuring cell with electrodes in a cavity and on a reference gas chamber having a defined, mostly higher oxygen concentration. This measuring cell typically displays a resulting Nernst voltage characteristic, which is distinguished by a sharp potential increase as soon as the oxygen concentration in the cavity sinks to zero. A pump current is regulated to a regulated setpoint value, so that a corresponding potential results within the potential increase at the measuring cell. The regulating setpoint value typically includes a Nernst voltage of 450 mV, which is used for the purpose of regulating an oxygen concentration in the cavity of λ=1. This regulating setpoint value is typically 450 mV over the entire service life of the lambda sensor. If the regulating setpoint value of the measuring cell changes within the sharp potential increase, for example, to 300 mV-600 mV, the oxygen concentration in the measuring chamber does not decisively change. The oxygen inflow and the pump current are thus hardly influenced.
Lambda sensors having only one cell are discussed, for example, in DE 2946440 A1. A method for obtaining a control variable for regulating the air-fuel ratio of the operating mixture of internal combustion engines with the aid of an exhaust gas measuring sensor exposed to the exhaust gas flow is provided in this unexamined published application.
In lambda sensors having only one cell, an outer electrode of the one cell, in particular a pump cell, is typically applied to a gas chamber having a high oxygen concentration, for example, to a reference volume. A fixed voltage is applied between the outer electrode and an inner electrode of the pump cell. As soon as an oxygen concentration in a cavity is close to 0, a potential, in particular a Nernst potential, increases strongly and partially compensates for the applied voltage. A constant oxygen concentration in the cavity may thus also be regulated in this way with good precision. For this purpose, the voltage at the pump cell must exceed an ohmic voltage drop of a pump current via a resistor of the pump cell. A sum of the ohmic voltage drop and the desired Nernst potential, which may typically be 450 mV, is ideally applied. The voltage should actually be somewhat higher to compensate for contact resistances at the electrodes.
As in the case of lambda sensors having two cells, a change of the voltage at the strong potential increase, for example, in the case of Nernst voltages between 300 mV and 600 mV, i.e., a change of the voltage by +/−150 mV to the typical value of 450 mV, does not result in any substantial change of the oxygen concentration in the cavity. An oxygen inflow from the measuring gas chamber via a diffusion barrier and the pump current are typically hardly influenced by voltage changes.
Due to a strongly varying oxygen concentration in the exhaust gas, the resulting pump current may be subjected to strong variations, the voltage being tracked in order to compensate for the changed ohmic voltage drop.
Lambda sensors, for example, broadband lambda sensors, are used in particular in the exhaust flow direction downstream from a NOx storage catalytic converter (NSC), in order to diagnose the NSC. A time is typically ascertained for this purpose using the lambda sensor, which elapses until a rich jump in a mixture formation breaks through the NSC, i.e., until the lambda sensor indicates a specific rich pump current.
A method for operating a broadband lambda sensor is discussed in patent specification DE 10216724 C1, in order to also maintain the measuring sensitivity of a sensor in the case of fuel post-injection during lean operation and/or in a “fast light off.” During the duration of a fuel post-injection and/or the “fast light off,” a pump voltage is repeatedly reversed in polarity, so that an anodic pump current briefly results, which pumps oxygen ions into a measuring chamber, which oxidize hydrocarbons therein.
It is understood to be legally required that the function of a lambda sensor be diagnosed and monitored over its lifetime, for example, to diagnose an NSC in a vehicle. It is understood that the characteristic of lambda sensors may change over longer operating times. For example, the relationship between an applied oxygen concentration in the exhaust gas and a resulting pump current of the sensor element changes. If the characteristic changes due to aging effects in the diffusion barrier, for example, due to a change in the material or clogging, the slope of the characteristic changes in particular. The fundamental curve profile is maintained, however. Such changes may be compensated for in principle by a compensation at a known measuring point, for example, in the case of operation using ambient air.
A gas sensor and a method for the operation thereof are discussed in DE 10163912 A1, in the case of which operating phases occur, during which the measuring gas, which communicates with a diffusion chamber of the gas sensor, corresponds to a reference gas and accordingly a λ value of the measuring gas is known at this point in time. These operating phases having a known λ value may be used according to this publication for the regular monitoring or calibration of measuring signals of the gas sensor. It may be provided for this purpose that the pump voltage to be applied to the pump electrodes of a pump cell is reversed in polarity in relation to a normal pump operation periodically and/or in predefined operating phases, so that polarization effects in a ceramic body are dissipated and changes of sensor signals connected thereto are prevented.
A forced pump current reversal for regenerating the pumping capability is discussed in both DE 10216724 C1 and DE 10163912 A1.
In addition to aging effects in diffusion barriers, the activity of the electrodes may also decrease, for example, by contamination due to additives in oil and fuel, for example, silicon and/or lead. For example, an applied pump voltage may then no longer be sufficient in the event of high oxygen concentrations in the exhaust gas, in order to apply a required pump current. The characteristic thus flattens out strongly in particular in the case of high oxygen concentrations. Such a change of the characteristic may no longer be compensated for. In the case of a lambda sensor having two cells, this effect could be recognized by monitoring the regulated regulating point, in particular the setpoint voltage, as soon as the maximum possible setpoint voltage is no longer sufficient. Since such aging effects slowly increase, a decisive signal corruption may thus already occur earlier. Since the sensor element is not exposed to rich or lean gases at a position downstream from the NSC, except during a diagnosis of the NSC, but rather is continuously located in a λ=1 atmosphere, a pump capability of the sensor element may not be tested. A method is needed for checking the functional reliability, in particular of a pump cell, in the case of λ=1 atmosphere. Known methods for diagnosing a sensor element, for example, the above-described conventional monitoring of the sensor signal at a known measuring point, for example, in the case of application of ambient air, are greatly restricted, since less critical aging effects and production scatter may also have similar effects in this case.